1. Field of the Invention
The present invention relates generally to phase plugs for loudspeakers and more particularly to acoustical phase plugs that can provide either a rectangular planar wavefront or a rectangular wavefront with a desired radius of curvature from an output aperture of the phase plug.
2. Background Art
Acoustic design in general, and loudspeaker design in particular, benefits in sound quality from transformation of the shape of the wavefront radiated from a given device, such as a transducer, or driver, from a spherical wavefront to a planar wavefront. When far enough away, a planar driver aperture can be almost considered to be a point source and the wave is experienced as a spherical wave. As result of the sound projection from a finite planar source, some diffraction occurs as a result of the size of the sound source. Different shapes or different boundary conditions that tend to confine the wavefront have been proposed in various ways in an effort to equalize the path lengths and provide for a planar rectangular wavefront at the exit aperture.
One such attempt was the use of a frusto-conical diaphragm design for a phase plug in U.S. Pat. No. 4,718,517 to Carlson, assertedly so as to provide a direct acoustic coupling of the cone type or apex driven loudspeaker to the entry of a rectangular horn. Similarly, Heil in U.S. Pat. No. 5,163,167 and Adamson in U.S. Pat. No. 6,095,279, each utilize a spreading cone having as a central element within a similar, cone-shaped cavity to transform a circular planar wavefront emitted by a compression driver into a rectangular planar wavefront. Both these patents include a section that begins as a cone, but transitions to a wedge shaped end created by surfaces that obliquely section through the conical surface, that is, with the cutting planes intersecting the diameter of the circular base for Heil or the major axis of the ellipse for Adamson.
Adamson in U.S. Pat. No. 6,581,719 teaches that for any horn type device to be considered a true waveguide, it must meet the criteria that the wavefront will always intersect the boundary of the waveguide at a 90 degree angle. Adamson also suggests in the patent that any boundary not normal to the wavefront will cause a reflection of energy, thus reducing contact with the waveguide wall. The ramification of this is that for opposing walls that diverge, the wavefront propagating through the horn must have some amount of curvature. Adamson '719 attempts to solve this problem of a curved wavefront by adding a second “wave shaping” chamber to a primary waveguide structure (in the shape of a simple horn). The simple horn acts to expand the sound wave to a circular or arcuate ribbon shape having a rectangular exit profile. In the separate second chamber, the arcuate sound wavefront is directed around an oblong shaped obstruction to provide a desired change, e.g. greater uniformity, in the different path lengths.
In another attempt to provide a uniformly rectangular planar wavefront at especially higher frequencies, as described by Heil in U.S. Pat. No. 5,163,167, a waveguide is placed at the output end of a compression driver to provide a transformative function and thereby to expand the wave from a circular planar surface, that is, a wavefront that is planar in cross-section with circular boundary constraints, to a rectangular planar wave surface, that is, a wavefront that is planar in cross-section with rectangular boundary constraints. Heil teaches a loudspeaker device having a compression chamber, with the device having a conduit with plural passages and two openings at the ends of the passages. One end is fitted to the output orifice of a compression driver, and the other end is the output orifice of the loudspeaker device. A planar, or isophase, circular wavefront is thus transformed at the other end, comprising the loudspeaker device output, so it emits a planar and oblong, and ideally, a planar rectangular isophase wavefront. Heil further describes the phase plug in the conduit as desirably providing passages for the propagation of sound energy such that the time interval between the input and output orifices remains at the shortest paths allowed within the passages are of practically equal length from the input orifice to the output orifice of the conduit. The device is said to improve at higher frequencies, particularly for frequencies with wave lengths less than approximately 15 cm.
Adamson teaches the use of a loudspeaker and chamber with a waveguide structure in several patents, including U.S. Pat. Nos. 6,095,279, 6,343,133, 6,581,719 and 6,628,796, and teaches devices that utilize an inner body as a central element within a similar shaped cavity to transform a circular planar wavefront radiated by a compression driver into a rectangular planar wavefront at the output of the device into a horn section. As described above, in U.S. Pat. No. 6,581,719, Adamson teaches use of two separate chambers, a primary waveguide which generates a rectangular cylindrical wavefront, and a separate second sound wave forming chamber that provides purposefully designed unequal pathlengths so as to transform the rectangular cylindrical wavefront to a rectangular planar wavefront. Adamson teaches that a rectangular planar wavefront is better suited to drive the input of certain horn designs, as well as for use in line array applications.
The surface in the devices disclosed by Adamson '279 differs from that of Heil in that the frusto-conical insert is not circular at its base, but is instead elliptical with the cutting planes intersecting the semi-major axis, instead of the diameter of a circular base. This allows for a path length along the middle of the surface to be slightly shorter than a path length along the top or bottom of the surface.
However, the Heil and Adamson '279 configurations both include discontinuities in the wave guide path that introduce a certain amount of diffraction and interference with the wavefront. These discontinuities generate unwanted diffraction, which affects the optimum quality of the sound as it is emitted from the output orifice and is projected into a horn or into free space. The parabola shaped transitional edge between the conical portion and the wedge portions of both Heil and Adamson give rise to diffraction of the sound wavefront caused by the discontinuities within the cavity formed by the inner body and outer shell. This leads to less than optimum performance of the device because of the resulting interference in the wavefront caused by the reflected sound within the cavity originating from the diffraction at the discontinuities. Diffraction of the sound wavefront is to be avoided to eliminate the possibility of detrimental interference. As described, Adamson '719 requires two separate chambers to transform a rectangular cylindrical wavefront to a rectangular planar wavefront, thereby increasing the overall length of the device and the pathlength which the sound waves must travel.
Other attempts have been made toward the same end, for example, in U.S. Pat. No. 6,650,760 to Andrews et al., U.S. Pat. No. 6,668,969 to Meyer, U.S. Pat. No. 7,177,437 to Adams, U.S. Pat. No. 7,510,049 to Kling, U.S. Pat. No. 7,631,724 to Onishi and U.S. Pat. No. 7,735,599 to Kubota. However, the above described attempts all suffer from similar problems as do the '279 Adamson and Heil devices, albeit some to a lesser extent.
The prior art patents to date teach configurations having some amount of discontinuities in the waveguide, or require at least two chambers to accomplish the transformation, thereby necessarily lengthening the dimension of the phase plug device. Thus, what is desired is a method for determining and transforming a uniform wavefront at an input aperture, guided through one or more passages, to produce a wavefront with a predetermined amount of curvature (or no curvature), as desired, at an output aperture. Ideally, the wavefront emitted from this configuration has little or no change to the spectral content of the wavefront at the output aperture compared to the input aperture. That is, it is desirable for constructive and destructive interference at various frequencies to be avoided. Also desirable is a true waveguide derived from the use of a single chamber device that transforms a circular planar wavefront to a rectangular planar wavefront, and provides continuity in the waveguide, avoiding any discontinuities or sharp angles. This ideally produces an isophase rectangular planar wavefront, or a wavefront with a desired amount of either convex or concave curvature, as it exits the output aperture of the phase plug device, and enters either a loudspeaker horn or the open acoustic space beyond the output aperture.